1. Field of the Invention
This invention relates to the inspection of glass articles for the detection of stress in surfaces thereof and is more particularly concerned with an auxiliary device which is adaptable to a light polarizing strain detector for facilitating inspection of glass ophthalmic lenses for the presence or absence of stress patterns resulting from ion exchange treatment.
2. Discussion of the Prior Art
In order to enhance the impact resistance of glass ophthalmic lenses so as to safeguard against injury to the eyes of a wearer and yet not require the lenses to be excessively thick, heavy and/or otherwise ungainly, it has become customary to treat the surfaces of these lenses in such manner as to induce a surface compressive stress therein. This may be accomplished by thermal tempering or chemical ion exchange treatment.
Thermally tempered glass is produced by rapidly cooling a glass piece which has been heated to near the softening point. This produces a compressive stress on the surface of the article while the interior of the article is under tensile stress.
Glass articles which have been strengthened by ion exchange treatment become compressively stressed by the substitution of a larger monovalent alkali metal ion for a smaller monovalent alkali metal ion in the surface of the glass article causing its surface to become more densely packed than its interior.
The terms "surfacse treatment", "surface treated" and variations thereof as used in this specification and its appended claims are intended to include all surface layers which may be used to strengthen the subject articles, regardless of the precise method by which such layers are formed. Layers of stress are herein considered as being surface treated layers whether produced by thermal tempering, chemical ion exchange, or otherwise.
Since neither of the aforesaid methods of surface treatment alter the appearance of the treated article (e.g. an ophthalmic lens) sufficiently so that the presence or absence of treatment may be readily ascertained visually and there is the need to determine the presence or absence of treatment, industry has heretofor been obliged to invest in complex and expensive inspection apparatuses or systems particularly when inspecting for the presence or absence of the more difficult to detect surface stresses produced by chemical treatment (ion exchange).
The present invention overcomes the need for complex and expensive testing apparatuses and has as a principal objective the provision of a simple and inexpensive but efficient auxiliary device for conventional light polarizing strain detectors which can render such detectors suitable for use in inspecting for the presence or absence of thin layers of stress in ophthalmic lenses which have been strengthened not only by heat treatment but more particularly those strengthened by the more difficult to detect ion exchange treatment.